1. Field of the Invention
The invention relates to an electrophotographic apparatus and, more particularly, to automatically determining and achieving optimum optical focus therein.
2. Description of the Prior Art
An electrophotographic apparatus, for example a copier using xerographic principles, focuses an illuminated image of an original document on a photoconductive surface. The surface is selectively charged and discharged in accordance with the image. Copies of the original result from the transfer of a developing material from this surface to the copy medium. A basic determinant of copy quality is therefore the accuracy of the image focus. Since the image is usually scanned onto the photoconductor by a moving reflector, another important factor in copy quality is the accuracy of the relationships among the scanning elements. Copiers often permit the copy to be smaller than the original, an affect obtained by optical manipulation. The accuracy of this manipulation will have a significant affect on copy quality. A document glass is normally used to position the original document. The placement of index marks on the glass relative to the image ultimately formed on the photoconductor surface significantly affects copy quality.
Assuming that the above factors affecting copy quality can be accurately adjusted and aligned initially, with time, the copy quality will, nevertheless, deteriorate due to mechanical wear, slippage, stretching and the like.
It is, therefore, evident that an accurate initial adjustment and alignment technique is essential to the manufacture of a high-quality copier and that if this technique is simple, the same technique will be useful in maintaining the operation of a copier.
Typically, adjustment of lenses, lights, carriages, fiber optic bundles, reflectors, mirrors, document glasses, etc. has been performed manually by skilled technicians. For example, the photoconductor surface may be replaced with a temporary screen for viewing the image during adjustment. The technician then adjusts the copier elements for the best apparent focus. Depending upon the complexity of the copier, the "best" focus may occur for different adjustments depending upon, for example, what part of the original document is scanned or whether the copy is being reduced. Among the problems with this approach is the difficulty of imitating the actual copier operation (such as scanning), the requirement that a "best" focus be selected from among several possible best foci, reliance upon the technician's subjective evaluation of focus and the normal human failings resulting from fatigue and inattention. In addition, this approach is slow and gives inconsistent, as well as inaccurate, results. As the copier gets smaller and more complex, it becomes even more difficult to quickly make the accurate adjustments necessary for an economically manufacturable copier.
The prior art discloses some attempts to minimize the manual steps involved in adjusting and aligning copiers. In U.S. Pat. No. 3,510,219, a variety of manual adjustments are facilitated by providing a leveling device for the document glass, a universally adjustable lens mounting, and the like. It is also known to place a television camera in a position which permits the technician to conveniently view the image essentially as it would appear on the photoconductor surface. Accuracy of adjustment and alignment has also been increased by substituting, during manufacture, a narrow beam of coherent light for the light source normally used during copying. None of these approaches eliminates all the manual steps and, therefore, to some extent, they all share the shortcomings inherent in humans.
Partially automated focusing and alignment systems are known in the prior art. In U.S. Pat. No. 3,623,790, a servo loop maintains a constant spacing between a lens and a film plane to both guarantee focus and avoid contact with the film. In U.S. Pat. No. 4,007,326, an electrical analog of a copy image is compared with an electrical analog of an original image. The copy and the original are adjusted until a comparison circuit connected to a television monitor causes the television images to cancel--indicating correct focus. U.S. Pat. No. 3,662,662 uses two optical systems to illuminate two photo cells. If one optical system is adjusted identically to the other (reference) system, both cells will be equally iluminated--indicating correct focus. All the foregoing require either human judgment, duplicate optics or rigidly-fixed adjustment parameters.
Attempts have been made to completely remove the subjective human element from the determination of focus. In U.S. Pat. No. 3,691,922, the point of sharpest focus occurs when a photocell detects a predetermined brightness/darkness distribution. In U.S. Pat. No. 3,593,286, an electronic image dissector tube is used to electronically scan an optical image and store electrical signals of the image for subsequently establishing similarities or changes between successive images. An electronic light sensor mechanically scans an image to determine the point of best focus in the Focatron Model P-122 marketed by Logetronics, Inc., Alexandria, Virginia. In the foregoing, changes in light distribution are analyzed and the plane of sharpest image is recognized when the largest number of target objects is recognized. An ITT Model F4100 Vidissector camera tube used in ITT Model F5019 camera unit marketed by the ITT Aerospace/Optical Division, Fort Wayne, Indiana, electronically scans an image and supplies signals indicative of image light levels. A suggested procedure for determining the sharpest vidissector camera focus includes the step of monitoring the video output for maximum white level and dark level response. In an IBM TECHNICAL DISCLOSURE BULLETIN article published July, 1972, pages 504-505, focusing is automatically adjusted as a function of a comparison between the widths of photocurrent pulses from light detectors.